1. Field of the Invention
The present invention generally relates to a digital signal reproducing apparatus for a PCM (Pulse Code Modulation) signal recorded on a recording medium such as a magnetic tape, and, more particularly to a digital signal processing apparatus where digital data recorded on the magnetic tape has been edited and the edited digital data is processed.
2. Description of the Related Art
The above-described digital signal recording and/or reproducing apparatus has been recently, widely developed such as digital audio tape recorders (DAT). In the known digital signal recording and/or reproducing apparatus, not only the editing process, e.g., the splice editing, or tape cut editing, but also the punch-in/punch-out operations are required. In the punch-in/punch-out operations, the digital signal having the continuous relationship with the original digital signal can be recorded on the original digital signal previously-recorded on the magnetic tape.
One of the conventional digital signal recording/reproducing apparatus is disclosed in, for instance, Japanese KOKOKU (examined) patent application No. 60-51176 (publication date: Nov. 12, 1985), which is capable of the editing process.
The circuit arrangement of the above-described conventional digital signal recording/reproducing apparatus will now be described.
In FIG. 1, there is shown a digital signal recording/reproducing apparatus including as the signal recording system, an analog signal input terminal 1, and an analog-to-digital (A/D) converter 2 for converting the analog signal derived from a signal source (not shown in detail) and supplied therefrom via the analog signal input terminal 1, into the corresponding digital signal. The apparatus shown in FIG. 1 further includes a Q-coding circuit 50 for performing the error-correction coding of the PCM (Pulse Code Modulation) signal, or data supplied from the A/D converter 2; a track distributing circuit 51 for distributing the PCM signal into a plurality of signal channels; an interleaving circuit 52 for delaying the respective PCM signals distributed in the multi-track with giving different delay times; a P-coding circuit 53 for adding error detecting code to a certain number of the respective track signals; a modulating circuit 9 for modulating the P-coded PCM signal to obtain the desirable digital signal (data); and a recording head 10 constructed by multi-track heads for recording the desirable digital signal on a magnetic tape 26.
In the signal reproducing system of the recording/reproducing apparatus, there are provided a reproducing head 11 for reproducing the digital signal (data) from the magnetic head tape 26; a demodulating circuit 12 for demodulating the signal reproduced from the reproducing head 11 to obtain the PCM signal; a P-code checking circuit 54 for checking the reproduced PCM signal; a first editing-point detector 15 for detecting the first editing point based upon the checking results of the P-code checking circuit 54; a de-interleaving circuit 55 for de-interleaving the PCM signal which has been interleaved in the signal recording system; and a track synthesizing circuit 56 for changing the multi-track PCM signal into a time sequential signal. The recording/reproducing apparatus further includes a Q-code decoding circuit 57 for performing the error correction for the time sequential signal. The digital signal recording/reproducing apparatus further includes a second editing-point detector 22 for detecting an editing point based upon the detection result of the Q-code decoding circuit 57; an editing process circuit 23 for electrically connecting the PCM signals adjacent to the editing point without causing a signal level jump in response to control signals derived from the first and secod editing-point detectors 15 and 22, respectively, a digital-to-analog (D/A) converter 24 for D/A-converting the edited digital signal data into a corresponding analog signal; and an analog signal output terminal 25 connected to the D/A converter 24.
FIG. 2 illustrates a data recording format of the magnetic tape 24 employed in the recording/reproducing apparatus. Reference numerals 60-1 to 60-8 indicate the recording tracks, i.e., eight (8) tracks. Assuming that the PCM signal train which has been A/D-converted and processed into a time sequential signal, is "Dn", several blocks of time delays for each track ("l" unit in FIG. 2) are given to this PCM signal train "Dn", and thus the delayed signals are arranged in the form as shown in FIG. 2. In the recording track 60-1, the error checking code (i.e., a cyclic redundancy check code, CRCC) "P" is added to the PCM signals D.sub.1, D.sub.7, - - - D.sub.37, and, furthermore, the sync signal "S" is superimposed therewith, thereby forming a single frame. It is obvious that the remaining recording tracks 60-2 to 60-6 have the same frame arrangements. However, in the recording track 60-7, the error checking code "P" is superimposed on check symbols Q.sub.1, Q.sub.3, - - - , Q.sub.13 of the error correction code (i.e., Reed-Solomon code) which has been added to a predetermined PCM signal train, and also the sync signal "S" is added thereto so as to form another frame. The signal arrangement of the recording track 60-8 is similar to that of the recording track 60-7. It should be noted that a combination of the recording tracks 60-1 to 60-8 is so-called as a "block".
For the sake of easy understanding, FIG. 3 illustrates arrangements of the PCM signal and the check symbol Q of the correction code. As previously described, the time-sequentially arranged PCM signal series D.sub.n are separately arranged with several l blocks as shown in FIG. 3. This is so-called as an "interleaving", which owns a first function to disperse errors caused by scratches and defects of the magnetic tape. This interleaving has another function in that both the PCM signal located before the editing point on the recorded digital signal data and the PCM signal located after the editing point are duplicated for a predetermined time period by employing correction means (not shown), and the digital data before the editing point is faded out whereas the digital data after the editing point is faded in so as to perform the signal cross-fade, whereby the digital signals before and after the editing point can be electrically connected to each other smoothly as to the signal levels.
Error correction code for the digital signal recording/reproducing apparatus shown in FIG. 1 will now be described. The error checking code "P" for the magnetic tape's longitudinal direction is equal to (200,224) code on GF(2), whereas the error checking code "Q" for the magnetic tape's transversal direction is equal to (8,6) Reed-Solomon code on GF(2.sup.4). First GF(2) corresponds to a Galois field having elements 0 and 1, and second GF(2.sup.4) corresponds to a Galois field having element of 2.sup.4. It should be noted that (n,k) code implies the code defined by the code length "n" and the information symbol "K". The P-code is CRCC and the generating polynominal is given as follows. EQU G(X)=X.sup.16 +X.sup.12 +X.sup.5 +1 (mod 2) (1)
Further, the parity check information of the Q code and the codes Q.sub.1 and Q.sub.2 are given by the below-mentioned equations. ##EQU1## where .alpha.i (i=1 to 6) is the element on GF (2.sup.3).
The multiply code for the P-code and Q-code owns the following correction capability.
(A). When there is no track error, the detection result implies no error in both the P-code and Q-code.
(B). When there is one track error, a detection is made which track contains the error by utilizing the Q-code, and thus the error correction is performed.
(C). When there are two track errors, these track errors are corrected by utilizing the information of the error track for the P-code and the information of the error track for the Q-code.
(D). Where there are more than three track errors, the PCM signal (i.e., the digital signal data) belonging to these erroneous tracks, is concealed by utilizing the track information for the P-code.
It should be understood that "error correction" means that the PCM signal of the erroneous track is returned to the original correct PCM signal, whereas "error concealment" implies that the PCM signal of the erroneous track is error-concealed by utilizing the PCM signals located before and after this erroneous PCM signal to produce a PCM signal analogous to the original correct signal.
Operations of the digital signal recording/reproducing apparatus, as illustrated in FIG. 1, will now be described.
The analog signal supplied to the analog signal input terminal 1 is A/D-converted into the corresponding PCM signal series by the A/D converter 2. In the Q-coding circuit 50, the checking symbols "Qn" produced by the previous equations (2) and (3) are added, or superimposed on this PCM signal series "Dn". Then, the PCM signal series is so arranged in the PCM signal arrangement by the track distributing circuit 51 and the interleaving circuit 52 as illustrated in FIGS. 2 and 3. In addition, the checking code of the CRCC codes produced by the generating polynominal as defined by equation (1), is added to the above-described PCM signal series of each signal track in the P-coding circuit 53. Furthermore, the sync signal "S" is also added to this PCM signal series for each signal track, and thereafter, the resultant PCM signal series is transferred to the modulation circuit 9. In the modulation circuit 9, the input PCM signal series is modulated by employing a proper modulation signal to obtain the modulated PCM signal series recordable on the magnetic tape 26. The recording magnetic head 10 records the modulated PCM signal series on the magnetic tape 26.
Reproducing operations for the digital signal recording/reproducing apparatus illustrated in FIG. 1 will now be described.
The digital data signal recorded on the magnetic tape 26 is reproduced by the reproducing magnetic head 11 and then converted into a PCM signal by the demodulating circuit 12. The PCM signal is error-detected in the P-code checking circuit 54, and thereafter, transferred via the de-interleaving circuit 55 and the track synthesizing circuit 56 to the Q-code decoding circuit 57. The PCM signal transferred to the Q-code decoding circuit 57 is again error-detected, and error-corrected therein to obtain a time-sequential PCM signal "Dn".
When reproducing no tape-cut editing, or splice editing tape 26, the PCM signal output from the Q-code decoding circuit 57 is directly derived from the editing process circuit 23 without signal processing and then, D/A-converted by the D/A converter 24 into the corresponding analog signal. The resultant analog signal is output from the analog signal output terminal 25.
Reproducing the tape-cut editing tape 26, the first editing-point detecting circuit 15 produces a first editing-point detecting signal when the P-code checking circuit 54 detects that the signal frame of each track simultaneously contains a plurality of track errors because the tape-cut editing is performed for cutting the tape in the vertical direction with respect to the travel (longitudinal) direction of the magnetic tape 26. On the other hand, reproducing the tape-cut editing point in the code, the second editing-point detector 22 produces a second editing-point detection signal when the continuity of the codes located before and after the editing point is not maintained and the P-code check result is not coincident with the error information obtained by the Q-code. The editing process circuit 23 judges existence of the editing point when the first editing-point detection signal is output and also the second editing-point detection signal is output during a predetermined time period just after outputting the first editing-point detection signal. Both the PCM signal before the editing point and the PCM signal after the editing point are duplicated for a given time period by utilizing the correction means, both the PCM signals are cross-faded smoothly so as to avoid the signal jump. The output from the editing process circuit 23 is D/A-converted in the D/A converter 24 into the corresponding analog signal which is then output from the analog signal output terminal 25.
The error correction is performed in the following manner by the conventional digital signal recording/reproducing apparatus with the above-described circuit arrangement.
FIG. 4 illustrates the data error condition before and after the tape-cut editing point. Symbols D.sub.1 and Q.sub.1 indicate the data before the editing point, whereas symbols D.sub.2, D.sub.3, D.sub.4, D.sub.5, D.sub.6 and Q.sub.2 denote the data after the editing point. When reproducing the magnetic tape 26, the error is detected by the P-code check circuit 54 due to the scratches caused by the tape-cut editing in the data D.sub.3 and Q.sub.1, and Q-code decoding circuit 57 performs the above-defined correction capability (C). That is to say, two track errors are corrected by utilizing the information (D.sub.3, Q.sub.1) of the error track by the P-code, and the error information by the Q-code. In this case, there is, however, a drawback that although the data D.sub.1 before the editing point has no relationship with the data D.sub.2 to D.sub.6 after the editing point, the data D.sub.3, Q.sub.1 is corrected based upon this data D.sub.1, resulting in the error correction. Since a large quantity of errors are produced around the tape-cut editing point on the digital data track, which causes a degree of the error detection by the P-code to be increased, and also the data located before and after the editing point and having no relationship with each other is handled as one piece of code, there are great possibilities to cause the error pattern as illustrated in FIG. 4.
With the above-described circuit arrangement of the digital signal recording/reproducing apparatus, both the error correcting function and the code discontinuities detecting function are required. The former function is to correct the errors produced by recording and reproducing the magnetic tape 26, whereas the latter function is to detect the code discontinuities (i.e., noncorrection data portion, or correction-prohibited data portion of the digital signal data) before and after the tape cut editing point. In other words, the error correction must be performed while the correction capability belonging to the code is maximized in the error correcting function, whereas similarly, the detection for the correction-prohibited data portion is surely performed. As a result, if the decoding algorithm for satisfying the former function is employed in the recording/reproducing apparatus, no detection is made for the correction-prohibited data portion of the codes over the tape-cut editing point, and therefore, correction is mistakenly performed. To the contrary, if another decoding algorithm for satisfying the code discontinuities detecting function is employed, the error correction capability is deteriorated although the correction-prohibited data portion is surely performed. Accordingly, problems may occur in the normal signal reproducing. That is to say, there is a trade-off relationship between the error correction function and the detection function of the correction-prohibited data portion of the digital signal data.
Furthermore, the digital signal recording/reproducing apparatus has another drawback that no error correction is performed during the punch in/out operations.
The present invention has been accomplished in an attempt to solve the above-described conventional drawbacks, and therefore, has an object to provide a digital signal recording/reproducing apparatus where, in the reproducing operation, the error correction can be performed while the error correcting capability belonging to the code is maximized, and simultaneously the editing-point detection is surely performed.